The present invention relates to magnetic resonance imaging pulse sequences, and more specifically, to techniques for generating T1ρ-weighted images.
In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field, and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within the subject of interest. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium directions, causing the spins to precess around the axis of their equilibrium magnetization. During this precession and during relaxation, RF fields are emitted by the spinning, precessing nuclei and are detected by either the same transmitting RF coil, or by a separate coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
The contrast of the images so produced may be controlled via one or more pulse sequences played out by the gradient and/or RF coils. For example, a pulse sequence may be configured to generate a T1-weighted image or a T2-weighted image, with the weighting being a result of the spin relaxation parameter that is either avoided or magnified. The weighting of different spin relaxation parameters may result in images having useful information about a given tissue, such as blood saturation, tissue density, macromolecular content, and so on. However, the successful implementation of the pulse sequences that allow the generation of weighted images may be highly sensitive to inhomogeneities in the field generated by the coils and/or inhomogeneities in the gross magnetic field.
As these inhomogeneities are mostly equipment-related, they are typically unavoidable. Further, while active or passive shimming may partially correct for these, pulse sequences that correct for or cancel out such inhomogeneities are desirable. However, current techniques for performing these corrections are often inadequate, such that they do not completely correct for field inhomogeneity, or are subject to further improvement.